A microelectromechanical system (MEMS) is a microdevice that integrates mechanical and electrical elements on a common substrate using microfabrication technology. The electrical elements are formed using known integrated circuit fabrication techniques while the mechanical elements are fabricated using lithographic techniques that selectively micromachine portions of a substrate. Additional layers are often added to the substrate and then micromachined until the MEMS device is in a desired configuration. MEMS devices include actuators, sensors, switches, accelerometers, and modulators.
MEMS switches have intrinsic advantages over conventional solid-state counterparts, such as field-effect transistor switches. The advantages include low insertion loss and excellent isolation. However, MEMS switches are generally much slower than solid-state switches. This speed limitation precludes applying MEMS switches in certain technologies, such as wireless communications, where sub-microsecond switching is required.
MEMS switches include a suspended connecting member called a beam that is electrostatically deflected by energizing an actuation electrode. The deflected beam engages one or more electrical contacts to establish an electrical connection between isolated contacts. When a beam is anchored at one end while being suspended over a contact at the other end, it is called a cantilevered beam. When a beam is anchored at opposite ends and is suspended over one or more electrical contacts, it is called a bridge beam.
The key feature of a MEMS switch that dictates its highest possible switching speed is the resonance frequency of the beam. The resonance frequency of the beam is a function of the beam geometry. The beams in conventional MEMS switches are formed in structures that are strong and easy to fabricate. These beam structures are suitable for many switching applications, however the resonance frequency of the beams is too low to perform high-speed switching.
FIG. 1 illustrates a prior art MEMS switch 10 that includes a cantilever beam 12. The beam 12 consists of a structural portion 14 and a conducting portion 16. High-speed MEMS switches require both strength and high conductivity making it necessary to use the composite beam 12. The MEMS switch 10 further includes an actuation electrode 18 and a signal contact 20 that are each mounted onto a base 22. One end 24 of the beam 12 is connected to the base 22 and the other end 26 of the beam 12 is suspended over the signal contact 20. The suspended end 26 of the beam 12 moves up and down when a voltage is applied to the actuation electrode 18. As the end 26 of the beam 12 moves up and down, the conducting portion 16 of the beam 12 engages and disengages the signal contact 20.
FIG. 2 illustrates the prior art MEMS switch 10 during fabrication. The MEMS switch 10 includes a release layer 28 that is removed by conventional techniques such as etching. Removing the release layer 28 exposes the actuation electrode 18, the signal contact 20, and the conducting portion 16 of the beam 12. The conducting portion 16 of the beam 12 and the contacts 18, 20 are usually made of the same acid resistant metal because they must withstand the process of removing the release layer 28. Gold is the most commonly used material for the conducting portion 16, the actuation electrode 18, and the signal contact 20.
The MEMS switch 10 typically needs to operate in excess of 10 billion switching cycles such that the repeated contact between the signal contact 20 and the conducting portion 16 of the beam 12 is a critical design consideration. There are many mechanisms that contribute to the aging and failure of contacts. These mechanisms include mechanical impact damage, sliding-friction induced damage, current-assisted interface bonding, solid-state reaction, and even local melting. When the conducting portion 16 and signal contact 20 are made of the same metal, they tend to bond each other such that the switch 10 oftentimes does not open at the appropriate time, especially if the contacts are made of a very soft material such as gold. Gold bonding can easily occur at room temperature such that the operating life of existing MEMS switches is typically below 1 billion switching cycles.